An optical communication system includes an optical transmitter for transmitting a signal light, an optical receiver for receiving the signal light from the optical transmitter, and an optical fiber for connecting the optical transmitter and the optical receiver. In such an optical communication system using a direct detection method, the optical transmitter is provided with a modulator for modulating a bias current in accordance with a data signal to provide a signal light (transmission light). On the other hand, the optical receiver is provided with a demodulator for demodulating a received signal light to reproduce the data signal.
Recently, in such an optical communication system, an optical FSK (frequency shift keying) and an optical PSK (phase shift keying) have been used as a signal modulating method. In a situation that FSK or PSK is used in an optical communication system, a heterodyne detection method can be also used for receiving a transmission light as well as a direct detection method.
According to an optical receiver using a heterodyne detection method, a wide bandwidth is required for receiver circuit. On the other hand, according to optical transmitter and receiver using a direct detection method, a signal light is directly modulated by a baseband signal and the baseband signal is directly detected, so that a high bit rate transmission can be carried out easily.
The optical receiver of a direct detection type includes an optical frequency discriminator or an optical phase discriminator to detect an FSK signal light and a PSK signal light.
One of optical frequency discriminators has been described on pages 376 to 377 in a "Electronics Letters", Vol. 26, No. 6, 1990. According to the letter, a signal light is demodulated in accordance with a transmission characteristics of a Mach-Zehnder interferometer. In this case, the interferometer is required to have a center frequency which is coincided with that of the signal light in order to increase a detection efficiency thereof. For the purpose of controlling the frequency of the interferometer, one of two waveguides of glass (SiO.sub.2) is heated by a heater to be adjusted in phase.
According to the conventional optical receiver, however, there are disadvantages in that a frequency of the interferometer is difficult to be coincided with the center frequency of the signal light at high speed, because the frequency of the interferometer is controlled by heating process. As a result, a detection efficiency is not stable sufficiently. Further, a receiving sensitivity of the conventional receiver is low.
Otherwise, an optical frequency discriminator for demodulating a signal light in accordance with a propagation delay-time difference between two intrinsic optical axes (principal optical states) of a PM (polarization maintaining) fiber has been described in a report, R. S. Vodhanel, "Frequency modulation response measurements to 15 GHz using a novel birefringent fiber interferometer", Tech. Dig. Topical Meeting Opt. Fiber Commun., pager WQ13, 1989. The frequency discriminator performs the same operation as the Mach-Zehnder interferometer as described above.